1. Field of the Invention
Embodiments of the present invention relate in general to an actuator for an optical pick-up, and, more specifically, to an actuator for an optical pick-up capable of accommodating a plurality of objective lens for recording and/or reproducing information onto and/or from a plurality of optical discs having different recording densities.
2. Description of the Related Art
A digital versatile disc (hereinafter, it will be abbreviated to DVD), as well known, uses a light having a wavelength of 650 nm (or 635 nm), and an objective lens with a numerical aperture of 0.6 (0.65 for a DVD RW) to record and/or reproduce information onto and/or from the disc. Suppose that a DVD has a diameter of 120 mm, and a track pitch of 0.74 μm. Usually, this type of DVD has a recording (or storage) capacity of greater than 4.7 GB for one side.
Unfortunately however, the DVD is not adequate as a recording medium for recording HD (High Definition) moving images. This is because to record HD moving images lasting, say, 135 minutes, at least 23 GB/side of recording capacity is required.
To meet the demand for a high-density recording capacity, blue light with a wavelength shorter than red light, and an objective lens with a numerical aperture greater than 0.6 are now used. In addition, greater efforts have been given to the development and standardization of a next generation DVD, or a high-definition optical disc having a narrower track (hereinafter, referred to as HD-DVD (High Definition-DVD)).
On the other hand, as an optical disc employs an objective lens with a larger numerical aperture, the thickness of the optical disc should be reduced in order to assure tilting tolerance of the optical disc. In consideration of an allowable tilting tolerance of the optical disc, the thickness of a DVD has been reduced to 0.6 mm, which is less than the thickness of a CD, 1.2 mm. It is possible that the thickness of a future HD-DVD may be reduced to 0.1 mm. Also, the numerical aperture NA of the objective lens has been increased from 0.45 for a CD to 0.6 for a DVD, while there is a high possibility that the NA of the objective lens for use in the HD-DVD will be as high as 0.85. As for a light source of the HD-DVD, a bluish-purple light source will be the best choice, considering the recording capacity requirement. A problem for the development of a new standard optical disc, though, is whether the new optical disc can be compatible with existing optical discs.
In effect, designing and manufacturing an objective lens having a high numerical aperture of 0.85, for example, involves very complicated technologies, and extending the working distance of the objective lens for use in the HD-DVD to be equal to the objective lens for use in the DVD is a difficult task.
To solve the above problem with the working distance, a compatible optical pick-up for high density recording/reproduction needs at least one objective lens for recording/reproducing information on/from a CD and/or DVD, and an additional objective lens having a higher numerical aperture for high density recording.
Meanwhile, an actuator for such an optical pick-up includes a magnetic circuit which enables the objective lens to drive in focusing and tracking directions. In the focusing direction the actuator ensures that the space between the optical disk and the objective lens is fixed, and in the tracking direction the actuator moves the objective lens to a desired tracking position (center of tracking).
However, the above-described optical pick-up utilizing a plurality of optical discs with different recording densities interchangeably requires objective lenses in one-to-one correspondence to each of the plurality of optical lenses. In general, the actuator for an optical pick-up with a plurality of objective lenses is installed in such a manner that the objective lenses loaded on a movable unit are able to move in focusing and tracking directions.
In this regard, the applicant of the present invention introduced, in Korean Laid-Open Publication No. 2003-0045259, an actuator for an optical pick-up shown in FIG. 1, which is capable of driving two objective lenses 1 and 2. Referring to FIG. 1, a bobbin 3 for holding the objective lenses 1 and 2, is movably supported by a holder 5 with the aid of suspension members 4. The bobbin 3 is driven in a focusing direction by a first magnetic circuit 6 having a focusing coil 6a and a focusing magnet 6b. Also, the bobbin 3 is driven in a tracking direction by a second magnetic circuit 7 having a tracking coil 7a, wound around the bobbin 3, and a tracking magnet 7b. Therefore, the actuator drives those two objective lenses 1 and 2 to record/reproduce information on/from a plurality of differing types of optical discs, each having different recording densities and thicknesses. More details on the optical pickup actuator illustrated in FIG. 1 can be found in Korean Laid-Open Publication No. 2003-0045259 and thus, will not be provided here.
However, the related art optical pick-up actuators having the above construction are often faced with a problem caused by low stiffness of the bobbin 3 holding the coils 6a and 7a. As a result, a shortage of a gain margin occurs relating to the resonance frequency of the secondary coil and secondary resonance.
In addition, the pairing of the magnetic circuits 6 and 7 and their magnets 6b and 7b, along with internal & external yokes, installed in the optical pick-up actuator, only makes the entire structure and mechanism of the optical pick-up actuator more complicated.